The invention relates to very thin, plateletlike titanium dioxide reduction pigments.
Plateletlike titanium dioxide reduction pigments are known. However, they are based on the use of mica as substrate. The titanium dioxide is precipitated onto the mica and is subsequently subjected to partial reduction to titanium suboxides. Reducing agents used are gaseous reducing agents, such as hydrogen and ammonia, or solid reducing agents, such as silicon or titanium. Some of these pigments also possess an additional top layer comprising silicon dioxide.
U.S. Pat. No. 4,948,631 describes a process for preparing particularly bluish pearl luster pigments by reducing titanium dioxide-coated mica pigments with ammonia at temperatures from 750 to 850.degree. C.
JP H4-20 031 describes a process for producing a colored mica pigment by mixing a titanium dioxide-mica pigment with titanium and reducing the resulting mixture under reduced pressure at from 500 to 1000.degree. C.
U.S. Pat. No. 4,623,396 describes a titanium dioxide-mica pigment consisting of mica, a first layer of titanium dioxide, titanium suboxides and titanium oxynitrides and a second layer of titanium dioxide.
Mica pigments are used widely in the printing and coating industries, in cosmetics and in polymer processing. They are distinguished by interference colors and a high luster. For the formation of extremely thin layers, however, mica pigments are not suitable, since the mica itself, as substrate for the metal oxide layers of the pigment, has a thickness of from 200 to 1200 nm. A further disadvantage is that the thickness of the mica platelets within a certain fraction defined by the platelet size in some cases varies markedly about a mean value. Moreover, mica is a naturally occurring mineral which is contaminated by foreign ions. Moreover, technically highly complex and time-consuming processing steps are required, including, in particular, grinding and classifying.
Pearl luster pigments based on thick mica platelets and coated with metal oxides have, owing to the thickness of the edge, a marked scatter fraction, especially in the case of relatively fine particle-size distributions below 20 .mu.m.
As a substitute for mica it has been proposed to use thin glass flakes which are obtained by rolling of a glass melt with subsequent grinding. Indeed, interference pigments based on such materials exhibit color effects superior to those of conventional, mica-based pigments. Disadvantages, however, are that the glass flakes have a very large average thickness of about 10-15 .mu.m and a very broad thickness distribution (typically between 4 and 20 .mu.m), whereas the thickness of interference pigments is typically not more than 3 .mu.m.
EP 0 384 596 describes a process in which hydrated alkali metal silicate is subjected at temperatures of 480-500.degree. C. to the action of an air jet, forming bubbles with thin walls; the bubbles are subsequently comminuted to give plateletlike alkali metal silicate substrates with a thickness of less than 3 .mu.m. However, the process is complex and the thickness distribution of the resulting platelets is relatively broad.
DE 11 36 042 describes a continuous belt method of preparing plateletlike or glitterlike oxides or oxide hydrates of metals of groups IV and V and of the iron group of the Periodic Table. In this method, a release layer comprising, for example, a silicone coating is first of all applied, if desired, to a continuous belt in order to facilitate the subsequent detachment of the metal oxide layer. Then a liquid film is applied which comprises a solution of a hydrolyzable compound of the metal which is to be converted into the desired oxide, and the film is dried and subsequently detached using a vibration device. The layer thickness of the platelets obtained is given as being 0.2 to 2 .mu.m, although no concrete examples of this are cited.
EP 0 240 952 and EP 0 236 952 propose a continuous belt method of preparing different plateletlike materials, including silicon dioxide, aluminium oxide and titanium dioxide. In this method, a thin liquid film of defined thickness of a precursor of the plateletlike material is applied, via a roller system, to a smooth belt; the film is dried and detached from the belt, forming plateletlike particles. The particles are subsequently, if desired, calcined, ground and classified.
The thickness of the platelets obtained in accordance with the method described in EP 0 240 952 is relatively well defined, since the film is applied very uniformly, via a roller system, to the continuous belt, for example. The layer thickness of the platelets is given in the examples as being 0.3 to 3.0 .mu.m. According to Example 1, a first roller is wetted with the precursor used by immersing this roller partially into a stock container which is filled with the precursor. The film is tranferred from this roller to a second, corotating roller which is in very close contact with the first roller. Finally, the film is rolled off from the second roller onto the continuous belt.
Disadvantages, however, are the use of very expensive precursor materials and, in particular, the increased requirements in terms of workplace safety which must be applied when organometallic compounds are used. The complete chemical conversion of the precursor into the desired layer material requires, in general, high heating of the film and of the belt material. In addition to the considerable thermal stress which this places on the belt material, the high energy consumption and the restriction on the process speed are highly disadvantageous for the economy of the method.
WO 93/08237 describes plateletlike pigments consisting of a plateletlike matrix comprising silicon dioxide, which may contain soluble or insoluble colorants and which is covered with one or more reflecting layers of metal oxides or metals. The plateletlike matrix is prepared by solidification of waterglass on a continuous belt.
DE 12 73 098 describes the preparation of a mother-of-pearl pigment by vapor deposition of ZnS, MgF.sub.2, ZnO, CaF.sub.2 and TiO.sub.2 films onto a continuous belt. This process, however, like the process described in U.S. Pat. No. 4,879,140 in which plateletlike pigments with Si and SiO.sub.2 layers are obtained by plasma deposition from SiH.sub.4 and SiCl.sub.4, is associated with very high expenditure on apparatus.
Despite numerous attempts, it has not hitherto been possible to develop any economic process for preparing very thin plateletlike titanium dioxide pigments having a layer thickness of less than 500 nm.